What Is Malignant?
Malignant describes a tumour or growth that is cancerous — it has the ability to invade surrounding tissues and spread (metastasise) to other parts of the body through the bloodstream or lymphatic system. A malignant diagnosis is more serious than a benign one, though the prognosis varies widely depending on the specific cancer type, grade, stage, and location.
Understanding Malignant
When a tumour is confirmed malignant through biopsy and histopathology, the veterinarian will typically recommend staging — a process of determining how far the cancer has spread. This usually involves chest X-rays, abdominal ultrasound, and lymph node sampling. Staging guides treatment decisions because a cancer confined to its original location (early stage) responds very differently to treatment than one that has already spread to distant organs (late stage).
Malignancy exists on a spectrum. Histopathologists assign tumours a grade (1, 2, or 3) that reflects how abnormal the cells look under the microscope and how aggressively the tumour is likely to behave. Grade 1 tumours behave more predictably; Grade 3 tumours are highly aggressive. The combination of tumour type, grade, and stage gives the most accurate prognostic picture.
Common malignant tumours in dogs include: mast cell tumours, osteosarcoma (bone cancer), haemangiosarcoma (blood vessel cancer — very aggressive), lymphoma, and mammary gland carcinoma. In cats: mammary carcinoma (90% malignant in cats vs. 50% in dogs), squamous cell carcinoma, and lymphoma are among the most common.